This invention relates to methods for chemically comminuting and increasing the heating value of low-rank coals. As utilized herein, the term "low-rank coals"are those coals having high oxygen and mineral matter contents, at least 15% in each case. Mineral matter in low-rank coals generally occurs in the range of 10-40% (by weight) and can comprise a variety of non-combustible inorganic constituents. Such mineral matter (also known as "ash") occurs as kaolinite and other clay minerals, quartz and gypsum. Sulfur occurs primarily as pyrite (FeS,), comprising about 40-80% of the total sulfur, and about 1-10% of the total coal.
Low-rank coals are relatively little used despite their relative abundance. Such coals have low heating value, high moisture content and high mineral matter content. Such coals are relatively common in Texas, Montana and North Dakota in large quantities, reasonably near the surface. Because of their abundance and accessibility, numerous attempts have been made to convert such coals into more environmentally-sensitive forms for use. For example, U.S. Pat. Nos. 4,313,737 and 4,421,722, both to Massey et al involve the pretreatment of coals by explosive comminution. Such process is intended to remove pyritic sulfur and other mineral inclusions from comminuted coal. In U.S. Pat. Nos. 4,775,387, Narain et al, a coal slurry in an aqueous alkali solution is heated under pressure to above the critical conditions of steam. Explosive release of the pressure fractures the coal into 10-30 micron particle size.
In U.S. Pat. No. 3,993,455, Reggel et al, pyritic sulfur and mineral matter is removed from coal by treating the coal with aqueous alkali at elevated temperatures followed by acidification with a dilute strong acid. Temperatures of from about 175-350.degree. C. and 6N sulfuric acid is preferred.
It has also been proposed that low-rank coals be biosolubilized in order to increase the heating value of coals prior to burning. Apparently, microbial coal solubilization results from production of alkali materials in situ, and can be enhanced with an oxidative pretreatment of the coal with various acids.
Low-rank coals generally have structures containing much more oxygen than do more commercially valuable, higher rank coals. Such oxygen is typically present as carboxylic acids, alcohols and ethers. To whatever extent a coal sample is soluble, usually in alkali solutions, the solubility is dependent upon the coal oxygen content--the greater the oxygen content, the greater the potential solubility. Solubilized coal has the added benefit of having less than 1% mineral matter (ash) associated with it, thereby removing as much as, if not more than 50% of the sulfur originally present in the coal. Therefore, it is an object of the present invention to provide a process whereby low-rank coals having high mineral matter (including sulfur) and oxygen contents can be solubilized, providing a finely ground comminuted coal having a reduced sulfur content.
Sulfur in coal is a major constituent of sulfur oxide (SO.sub.x) emissions which ultimately contribute to acid rain therefore, much effort is expended to remove sulfur from coal prior to burning. One process for separating mineral matter from coal, or desulfurizing coal, is to solubilize the coal using dilute alkaline solution, remove the mineral matter by centrifugation, and precipitate the solubilized coal with excess amounts of acid (see Olson at al, Fuel 67:1053 (1988)). While such process may be effective, it is not commercially practicable on the large quantities of coals burned for electric power generation.